MWFs are used as lubricants and coolants in metal cutting and grinding, and in drilling operations in industrial manufacturing, for example in automotive engine, transmission and stamping plants. MWFs come in three types: synthetic, semi-synthetic and oil-based (including mineral, vegetable and animal oils). They are typically formulated to include chemicals that inhibit metal corrosion and inhibit microbial activity (biocides). Over time the MWF degrades as it is used in machining operations and will eventually need to be disposed of. The disposal of spent MWF into the environment is very difficult due to a number of factors, including: (1) the high toxicity of the spent MWF caused by for example, biocides and other chemical components that may be added to improve the performance of the MWF; and (2) the high chemical oxygen demand (COD) of spent MWF.
COD is a measure of how much oxygen would be necessary to oxidise the components of materials—such as waste effluents—and is generally considered to be a measure of the organic content of such materials. Typically, the tolerated level for wastewater COD for disposal to the public sewer in the UK is around 2000 mg/L although this may be a higher or lower number depending on the local conditions and may also be a higher or lower number in different countries. Methods for measuring COD are well known in the art. One exemplary method is described by van der Gast & Thompson (2005) Biotechnology & Bioengineering 89, 3 357-366 in which a LASA 100 mobile laboratory photometer is used with COD cuvette test kits. The MWF samples in which COD content is measured are pre-filtered using a 0.2 μm pore-size membrane (Millipore, UK).
Due to the toxic nature and high COD of spent MWF, the discharging of the effluent into the environment is tightly regulated, particularly in the US and Europe.
Traditionally chemical and physical methods—such as ultra-filtration and flash/vacuum evaporation—have been used in the treatment of spent MWFs before their disposal into the environment. However these methods can be energy intensive, difficult to scale up for large volumes and are unable to treat the pollution loads of modern MWFs. One method for dealing with the pollution loads that are not removed by filtration is to follow the chemical or physical step(s) with a biological treatment of the spent MWF.
One method for the treatment of MWFs comprises the biological treatment of MWFs in which micro-organisms are added thereto to digest the unwanted constituents. Such methods of bioremediation of MWFs are often unable to reduce the COD sufficiently without the initial processing thereof—such as filtration or ultra-filtration of the spent MWF (see, for example, van der Gast & Thompson (2005) Biotechnology & Bioengineering 89, 3 357-366), which adds considerable time, inconvenience and expense to the biological methods. Other biological methods that have been described to bring about some reduction in COD have utilised a liquid inoculation of microorganisms into a bioreactor, wherein the microorganisms are capable of reducing the COD content of spent MWF. For example, Muszynski & Lebkowska (2005) Polish Journal of Environmental Studies 14, 1 p 73-79 and Hila et al. (2005) Journal of Chemical Technology and Biotechnology (2005) 80, 641-648 describe the selection and culturing of microorganisms from spent MWF and their subsequent liquid inoculation into a bioreactor. Another biological method that has been described to bring about some reduction in COD is to use stable communities of microorganisms that maintain their composition throughout the treatment process using a defined consortium of microorganisms. In this regard, W02008/102131 describes the use of a consortium of microorganisms consisting of at least an Agrobacterium spp., a Comamonas spp., a Methylobacterium spp. and a Microbacterium spp. for treating spent MWF. The methods described therein utilise a liquid inoculation of the consortium of microorganisms into a bioreactor and the use of ultrafiltrated MWF is also disclosed.
The present inventors have sought to develop methods for significantly reducing the COD content of spent MWF, suitably, on an industrial scale. However, in doing so, they encountered a number of problems. For example, they found that bioreactors established using the methods of the prior art typically require lengthy commissioning times before they are operationally effective for treating the spent MWF. This has serious time and cost implications for treating spent MWF on an industrial scale. By way of further example, they also found that bioreactors established using the methods of the prior art could often show erratic performance with some trials showing that the COD content was somewhat reduced whereas other trials showed poor levels of COD reduction. They also found that the reduction in COD that could be achieved with these methods was not low enough for their needs and so the MWFs would require further downstream treatment before being sent to waste, thereby adding further expense and inconvenience.
The present invention provides improvements in the biological treatment of spent MWFs and aims to overcome the problems associated with the prior art.